1. Field of the Invention
The present invention relates to an external memory for an electronically controlled sewing machine and, more particularly, to such an external memory for converting compressed stitch data for stitch patterns into driving data for a driving motor provided in the sewing machine and outputting the driving data to the sewing machine or for converting compressed display data to be used for pattern display into dot data to be displayed on a display and outputting the dot data to the sewing machine.
2. Description of Related Art
In a conventional electronically controlled sewing machine, stitch data for a plurality of stitch patterns, such as character patterns and figure patterns corresponding to code numbers of the stitch patterns, are preliminarily stored in a nonvolatile memory such as a ROM provided in a control device, and a desired stitch pattern selected by operating a pattern select key, such as a ten-key, is sewn on a work fabric.
In connection with a recent electronically controlled sewing machine having been practically applied, a ROM card (pattern card), for example, is prepared to store stitch data for many stitch patterns in every kind of pattern, so as to sew many stitch patterns of various kinds. The ROM card storing a desired stitch pattern is adapted to be mounted to a body of the sewing machine. The desired stitch pattern in the ROM card is selected to read stitch data for this stitch pattern, and the stitch pattern is sewn on a work fabric. In this case, the ROM card preliminarily stores stitch data only, and a control program for reading the stitch data is stored in a ROM provided in a control device of the sewing machine.
There has been proposed in U.S. Pat. No. 4,388,883 that a stitch pattern is divided into many blocks and stitch data of the stitch pattern is stored in a compressed condition from position data of apexes of the blocks and the number of stitches in the blocks. Accordingly, it is considered that the stitch data in the ROM card may be stored in a compressed condition by various compression techniques, so as to store many stitch patterns.
In the case where the stitch data of stitch patterns are stored in a compressed condition in the ROM card, and where the control program for reading the stitch data is stored in the ROM of the control device in the sewing machine, it is necessary to identify a data structure of the stitch data of stitch patterns stored in the ROM of the control device in the sewing machine with a data structure of the stitch data of stitch patterns stored in the ROM card. Therefore, compression of the stitch data of stitch patterns stored in the ROM card or change of the compressed condition in the ROM card is limited by the data structure of the stitch data stored in the ROM of the control device, that is, by the data read control program stored in the ROM of the control device. Furthermore, when the data structure of the stitch data stored in the ROM of the control device in the sewing machine is changed, it is necessary to change the data structure of the stitch data stored in the ROM card.